Control system for continuously variable transmission

ABSTRACT

A system for controlling a continuously variable transmission is disclosed, in which a shift actuator is controlled in a feed-back manner with a feed-back gain such that a deviation between target and current values related to a reduction ratio reduces toward zero, and the feed-back gain is altered in response to a current torque detected.

BACKGROUND OF THE INVENTION

The present invention relates to a control system for a continuouslyvariable transmission.

JP-A No. 63-43837 discloses a conventional control system for acontinuously variable transmission. This control system is constructedand arranged so that a target reduction ratio is achieved by a feed-backcontrol of a shift actuator. When a motor vehicle is not driven, viz.,an engine brake is applied, a feed-back gain becomes larger. Thisameliorates the followability and the responsibility of shifting to themaximum reduction ratio. Thus, even if quick deceleration takes place,shifting up to the maximum gear ratio can be achieved before the motorvehicle comes to a standstill.

A problem encountered in such a conventional control system for acontinuously variable transmission is that, since the feed-back gain iskept constant regardless of change in an input torque to thetransmission, viz., an engine torque, time is needed to bring a currentreduction ratio to the target reduction ratio when the engine torque islarge, and it is difficult to keep the target reduction ratiocorresponding to the current reduction ratio even during normaloperation of the motor vehicle. Specifically, with a continuouslyvariable V-belt transmission, the reduction ratio is determined by abalance between a hydraulic pressure on a movable conical member of apulley and a tension of a V-belt. If the engine torque increases out ofa predetermined balanced reduction ratio, for example, the tension ofthe V-belt also increases. In this event, the hydraulic pressure on thepulley increases in a delayed manner, the reduction ratio is deviated tothe maximum reduction ratio. Further, with a continuously variabletraction roller transmission which is disclosed, for example, in JP-UNo. 63-84451, shifting takes place in a controlling position of rollersupport members by the hydraulic pressure. Thus, when the engine torqueincreases, the hydraulic pressure within a hydraulic servo apparatus forposition control of the roller support members changes in a delayedmanner, resulting in an occurrence of a deviation between the currentreduction ratio and the target reduction ratio. Even during normaloperation of the motor vehicle, the hydraulic pressure for holding theroller support members has to increase as the engine torque becomeslarger. In this event, the hydraulic servo apparatus may not be held ina predetermined position due to increased leakage from seal portions,resulting in the occurrence of a deviation of the current reductionratio to the maximum reduction ratio.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a control system for acontinuously variable transmission wherein quick responsibility and highfollowability of shifting can be achieved.

According to one aspect of the present invention, there is provided asystem for controlling a continuously variable transmission such thatthe continuously variable transmission is adjusted to a reduction ratiothat is determined in response to a position which a shift actuator ispositioned at, the system comprising:

means for determining a target value of a predetermined variable relatedto a reduction ratio which the continuously variable transmission shouldbe adjusted to and generating a target value indicative signalindicative of said target value determined;

means for detecting a current value of said predetermined variable andgenerating a current value indicative signal indicative of said currentvalue detected;

means for detecting a current torque which the continuously variabletransmission is subject to and generating a current torque indicativesignal indicative of said current torque detected;

means for controlling the shift actuator in a feed-back control mannerwith a feed-back gain such that a deviation between said current valueindicative signal and said target value indicative signal reduces towardzero; and

means for altering said feed-back gain in response to said currenttorque indicative signal.

According to another aspect of the present invention, there is provideda method of controlling a continuously variable transmission such thatthe continuously variable transmission is adjusted to a reduction ratiothat is determined in response to a position which a shift actuator ispositioned at, the method comprising the step of:

determining a target value of a predetermined variable related to areduction ratio which the continuously variable transmission should beadjusted to and generating a target value indicative signal indicativeof said target value determined;

detecting a current value of said predetermined variable and generatinga current value indicative signal indicative of said current valuedetected;

detecting a current torque which the continuously variable transmissionis subject to and generating a current torque indicative signalindicative of said current torque detected;

controlling the shift actuator in a feed-back control manner with afeed-back gain such that a deviation between said current valueindicative signal and said target value indicative signal reduces towardzero; and

altering said feed-back gain in response to said current torqueindicative signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating a continuously variable tractionroller transmission to which a preferred embodiment of the presentinvention is applied;

FIG. 2 is an enlarged sectional view illustrating a continuouslyvariable traction roller transmission unit;

FIG. 3 is a longitudinal sectional view taken along the line III--III ofFIG. 2;

FIG. 4 is a schematic diagram illustrating a hydraulic circuit for thecontinuously variable traction roller transmission;

FIG. 5 is a block diagram showing an electrical connection of a controlunit;

FIGS. 6 to 9 are flowcharts showing a control program;

FIG. 10 is a graphical representation showing a relationship between aconstant K₁ and an engine torque T_(E) ; and

FIG. 11 is a view similar to FIG. 10, but showing a relationship betweena constant K₂ and the engine torque T_(E).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Referring to the accompanying drawings, a preferred embodiment of acontrol system for a continuously variable transmission according to thepresent invention will be described.

Referring first to FIG. 1, there is shown a continuously variabletransmission. A torque converter 12 is connected to a drive plate 10which is integrated with an output shaft of an engine. The torqueconverter 12 includes a lock-up clutch 12a which allows mechanicalconnection and disconnection of a pump impeller 12c on the input sidewith a turbine runner 12d on the output side by controlling a hydraulicpressure within a lock-up hydraulic chamber 12b. An oil pump drive shaft87 is connected to a cover 12e of the torque converter 12, and it isalso connected to an oil pump 15. The oil pump 15 and the torqueconverter 12 are arranged on the opposite sides of a continuouslyvariable traction roller transmission unit 16. The turbine runner 12d ofthe torque converter 12 is connected to a hollow input shaft 14.Connected to the input shaft 14 is the continuously variable tractionroller transmission unit 16 which includes an input traction disc 18, anoutput traction disc 20, and a traction roller 22 for transmittingtorque from the input disc 18 to the output disc 20. The input andoutput traction discs 18, 20 have toroid surfaces which come in contactwith the traction roller 22. An inclination of a shaft 80 of thetraction roller 22 is adjustable by a mechanism which will be describedhereinafter in connection with FIG. 3. The input traction disc 18 iscoupled with the input shaft 14, whereas the output traction disc 20 iscoupled with a gear 26 for unitary rotation. The gear 26 is inengagement with a gear 30 which is integrated with one idler shaft 28.Arranged to the idler shaft 28 are a gear 32 which is always rotatabletherewith, and a gear 34 which is rotatably supported thereon. By meansof a reverse clutch 36, the gear 34 is connectable with the gear 30 forunitary rotation. The idler shaft 28 is rotatable only in a forwarddirection, and thus not rotatable in a reverse direction by a one-wayclutch 31 which is mounted to a casing 67. This prevents thecontinuously variable traction roller transmission unit 16 from beingrotated in a direction contrary to that of the engine by an inversedriving force which is induced by road wheels. Another idler shaft 38 isarranged in parallel to the idler shaft 28A. A gear 40 is rotatablysupported on the idler shaft 38, and a gear 42 is always connectedthereto for unitary rotation. By a forward clutch 44, the gear 40 isconnectable to the idler shaft 38 for unitary rotation. The gear 40 isin engagement with the gear 32, whereas the gear 42 is always inengagement with a final gear 48 to which a pair of pinion gears 52, 54of a differential gear 50 are mounted. The pair of pinion gears 52, 54are in engagement with a pair of side gears 56, 58, each being connectedto the output shaft. With such structure, the output shaft rotates inthe forward direction by meshing the forward clutch 44, whereas itrotates in the reverse direction by meshing the reverse clutch 36. Inthe continuously variable traction roller transmission unit 16, areduction ratio is changeable continuously by controlling the conditionunder which the traction roller 22 contacts with the input traction disc18 and the output traction disc 20.

Referring to FIG. 2, the input shaft 14 is rotatably supported on thecasing 67 through a ball bearing 65 and a needle bearing 66. Between theinput shaft 14 and the ball bearing 66 is provided a spacer 68, whereasbetween the input shaft 14 and a loading nut 69 meshed therewith isprovided a belleville spring 70. The reaction of the belleville spring70 biases the input shaft 14 to the right as viewed in FIG. 2. A pin 71from the loading nut 69 falls into a groove 14a of the input shaft 14 totighten the loading nut 56. For receiving the pin 71, the loading nut 69is formed with a plurality of bores 69a, and also the input shaft 14 isformed with a plurality of grooves 14a. By receiving the pin 71 into anymated pair of the bore 69a and the grooves 14a, a position of theloading nut 69 relative to the input shaft 14 can minutely be adjusted.The pin 71 is locked by a bolt 72 to prevent disconnection. The outputtraction disc 20 is rotatably supported on the input shaft 14 through abearing 73. By means of a pair of keys 74 which are symmetricallypositioned, the output gear 26 is secured to the output traction disc 20for unitary rotation. Additionally, the input traction disc 18 isrotatably and axially slidably supported on the input shaft 14. A camflange 77 is arranged on the back of the input traction disc 18, viz.,on the opposite side of the output disc 20, and splined to the inputshaft 14. A shoulder 78 of the input shaft 14 inhibits the cam flange 77from sliding to the left as viewed in FIG. 2. A cam roller 79 isarranged between two opposite cam faces 18a, 77a of the input tractiondisc 18 and the cam flange 77. The cam roller 79 and the cam faces 18a,77a are shaped in a manner such that relative rotation between the camflange 77 and the input traction disc 18 causes a force to press theinput disc 18 to the right as viewed in FIG. 2. Rotatably supported onthe shaft 80 through a bearing 81 is the traction roller 22 which isdisposed in a toroid groove formed by two opposite faces of the inputand output traction discs 18, 20. Additionally, the traction roller 22is supported in a thrust direction by a ball bearing 82 which is in turnsupported by a roller support member 83. To prevent disconnection, thetraction roller 22, the ball bearing 82, and the roller support member83 are locked by two snap rings 84, 85 which are arranged to the shaft80 at both ends thereof. Inserted in a bore of the input shaft 14 is asleeve 86 which is locked by a snap ring 97 to prevent disconnection.The sleeve 86, except both end portions thereof to which an O-ring isarranged respectively, has the diameter smaller than the inner diameterof the input shaft 14, and a hydraulic passage 88 is defined by aclearance between the two. The input shaft 14 is formed with four radialbores 94, 93, 92, 91 to allow fluid communication with the hydraulicpassage 88, and also with a groove 101 and a bore 102 to receivehydraulic fluid out of a hole 90 of the casing 67. The groove 101 issealed by a seal ring 103. An oil pump drive shaft 87 is inserted in abore of the sleeve 86, and a hydraulic passage 89 for lock-up control ofthe torque converter 12 is defined by a clearance between the two.

Referring to FIG. 3, the right half as viewed in FIG. 3 is substantiallythe same as the left half as viewed in FIG. 3, so that a descriptionwill principally be made with regard to the right half. The rollersupport member 83 is rotatably and vertically slidably supported at theupper and lower rotation shafts thereof 83a, 83b by two sphericalbearings 110, 112. The spherical bearing 110 is supported by a bearingsupport member 114 which is in turn supported by a link post 116 securedto the casing 67. Similarly, the spherical bearing 112 is supported by abearing support member 118 which is in turn supported by a link post 120secured to an upper control valve body 200. The valve body 200 ismounted to the casing 67. The roller support member 83 has an extensionshaft 83c which is concentric with the rotation shaft 83b. It is to benoted that the extension shaft 83c is not formed with the rotation shaft83b, but is a member secured thereto. A piston 124 is arranged on theperiphery of the extension shaft 83c, and inserted in a cylinder 126which is formed inside the upper control valve body 200. A hydraulicchamber 128 is formed over the piston 124, whereas a hydraulic chamber130 is formed under the piston 124. Via a hydraulic passage defined by abore 203 of the piston 124, a clearance 204 between the piston 124 andthe extension shaft 83c, and two bores 206, 208 of the roller supportmember 83, the hydraulic chamber 130 is in fluid communication with anopening of the bore 208. It is to be noted that an opening of the bore206 is stopped by a ball 210. A race 212 of the bearing 82 is formedwith a bore 214. The left roller support member 83 includessubstantially the same hydraulic passage defined by the piston bore 203,the clearance 204, and the two bores 206, 208. Here, it is to be notedthat the bore 303 is in fluid communication with the upper hydraulicchamber 128, and that the bores 206, 208 are fluidly communicated witheach other via an annular groove 316. The right and left pistons 124 arethe same in shape, but have the bores 203 which are positioneddifferently to each other. Through a spacer 134, the piston 124 is incontact at the lower end thereof with a cam 136 which is secured, forunitary rotation, to the extension shaft 83c by a bolt 138. It is to benoted that the left extension shaft 83c is not provided with the cam136. For the shaft 80, a portion 80a to support the traction roller 22is eccentric with a portion 80b to be supported by the roller supportmember 83. The cam 136 includes an inclined surface 140 with which alink 142 is contact, so that rotation of the cam 136 causes the link 142to swing. A lower control valve body 144 is secured to the upper controlvalve body 200 at the bottom thereof through a separate plate 202. Forreceiving the lower control valve body 144 and the cam 136, an oil pan146 is attached to the casing 67. Secured to the lower control valvebody 144 is a shift control valve 150 which includes a drive rod 154which is rotated by a shift motor 152, a sleeve 156, a spool 158 whichis inserted in a bore of the sleeve 156, and a spring 160 which pressesthe spool 158 to the right as viewed in FIG. 3. The drive rod 154 has atthe leading end thereof an externally threaded head portion 154a whichis engaged with an internally threaded bore 156a of the sleeve 156. Thesleeve 156 has an axial groove 156b in which is received a pin 162secured to the lower control valve body 144. This allows the sleeve 156to be axially slidable without rotation. A force of the spring 160presses a right end 158a of the spool 158 to the link 142. The spool 158includes two lands 158a, 158b by which an opening degree of a portfluidly communicating with hydraulic passages 166, 168 is adjustable.When the reduction ratio is constant, the spool 158 is always located ata predetermined position in the sleeve 156 as shown in FIG. 3, and itoperates to supply the hydraulic fluid having the same level of pressureto the hydraulic passages 166, 168. On the other hand, during shifting,the spool 158 is located at a position other than the predeterminedposition, and it operates to distribute the hydraulic fluid out of thepassage 164 to the passages 166, 168 in accordance with that position.The hydraulic passage 168 is connected to the right and left hydraulicchambers 128, 130, whereas the hydraulic passage 166 is connected to theright and left hydraulic chambers 130, 128.

Referring to FIG. 4, a hydraulic control circuit includes the shiftcontrol valve 150, a line pressure regulator valve 502, a throttle valve504, a manual valve 506, a lock-up control valve 508, two constantpressure regulator valves 510, 512. These valves are connected to eachother as shown in FIG. 4. The circuit also includes an oil pump 15, ahigh-pressure chamber 516 (the right and left hydraulic chambers 130,128 in FIG. 3), a low-pressure chamber 518 (the right and left hydraulicchambers 128, 130 in FIG. 3), a forward clutch 44, a reverse clutch 36,an apply chamber 12f of the torque converter 12, a release chamber 12bof the torque converter 12, a solenoid 528, an oil cooler 530, and alubrication circuit 532, which are connected to each other as shown inFIG. 4. The line pressure regulator valve 502 adjusts a level ofhydraulic pressure of the hydraulic passage 534 (line pressure) to whicha delivery pressure of the oil pump 15 is supplied. The throttle valve504 produces a hydraulic pressure (throttle pressure) in response to aforce of a vacuum diaphragm 536, and supplies it to a hydraulic passage538. The shift control valve 150 controls, as described hereinbefore,the distribution of hydraulic pressure between the high-pressure andlow-pressure chambers 516, 518 by operation of the shift motor 152, andthus achieves a predetermined reduction ratio. For carrying out ashifting between forward and reverse drives of the motor vehicle, themanual valve 506 supplies the line pressure out of the hydraulic passage534 to the forward clutch 44 or the reverse clutch 36 in accordance withthe position of a select lever. In response to a hydraulic pressureproduced by the solenoid 528 which is controlled in duty ratio, thelock-up control valve 508 controls a distribution and a level ofhydraulic pressure for the apply and release chambers 12f, 12b forengagement and release of the lock-up clutch 12a. The constant pressureregulator valve 510 adjusts a level of constant pressure for thesolenoid 528, whereas the constant pressure regulator valve 512 adjustsa hydraulic pressure to the torque converter 12 within a predeterminedlevel.

Referring to FIG. 5, a control unit 300 which controls operation of theshift motor 152 and the solenoid 528 includes an input interface 311, areference pulse generator 312, a central processing unit (CPU) 313, aread-only memory (ROM) 314, a random-access memory (RAM) 315, and anoutput interface 316 which are connected to each other by an address bus319 and a data bus 320. Inputted to the control unit 300 directly or viathree wave shapers 308, 309, 322, and an analog-to-digital (A/D)converter 310 are signals from an engine revolution speed sensor 301, avehicle speed sensor 302, a throttle opening degree sensor 303, a shiftposition switch 304, a turbine revolution speed sensor 305, an enginetorque sensor 306, a brake sensor 307, and a maximum reduction ratiodetection switch 800. On the other hand, outputted from the control unit300 via an amplifier 317 and four lines 317a to 317d are signals to theshift motor 152 and the solenoid 528.

Referring to FIGS. 6 to 9, the content of control carried out by thecontrol unit 300 will be described.

Referring to FIG. 6, there is shown a program for complete engagementand lock-up controls preformed by the solenoid 528. This program isdisclosed, for example, in U.S. Pat. No. 4,735,113 which is hereinincorporated for reference.

Referring to FIG. 7, at a step 602, it is determined whether a vehiclespeed V is less than a predetermined value V_(O) or not. Thispredetermined value V_(O) is set equal to 2 to 3 km/h, for example. IfV<V_(O), a creep control is carried out as follows: At a subsequent step604, it is determined whether a throttle opening degree TH is less thana predetermined value TH_(O) or not. If TH≧TH_(O), viz., the throttlevalve 504 does not fall in an idle condition, the control proceeds to astep 606 where the duty ratio is set to 0%. By this operation, theforward clutch 44 is completely engaged. At a subsequent step 608, atarget pulse number P_(D) of the shift motor 152 is set to P₁. Then, thecontrol proceeds from the step 608 to a step 630 as shown in FIG. 9.

At the step 604, if TH<TH_(O), viz., the throttle valve 504 is in theidle condition, the control goes to a step 610, as shown in FIG. 9,where it is determined whether the maximum reduction ratio detectionswitch 800 is ON OR not. If the detection switch 800 is ON, the controlgoes to a step 612 where a deviation is obtained by subtracting a targetdeviation N_(m2) by a difference N_(D) between an engine revolutionspeed N_(E) and a turbine revolution speed N_(T), and set as e. At asubsequent step 614, a feed-back gain G₂ is retrieved based on thedeviation e. The duty ratio is set, at a step 616, in response to thedeviation e and the feed-back gain G₂. Subsequently, at a step 618, thetarget pulse number P_(D) of the shift motor 152 is set to 0 (zero),then the control proceeds from the step 618 to the step 630.

Referring again to FIG. 7, at the step 602, if V≧V_(O), a shift controlis carried out as follows: At a subsequent step 707, it is determinedwhether a current drive position is in "D" range or not. If the answerto the inquiry at the step 707 is YES, the control proceeds to a step902 where a "D" range target turbine revolution speed TRPM is determinedby retrieval of a "D" range target turbine revolution speed table. Onthe other hand, if the answer to the inquiry at the step 707 is NO, thecontrol goes to a step 709 where it is determined whether the currentdrive position is in "L" range or not. When the current drive positionis in "L" range, the control proceeds from the step 709 to a step 904where the "L" range target turbine revolution speed TRPM is determinedby retrieval of a "L" range target turbine revolution speed table. Onthe other hand, when the current drive position is not in "L" range, thecontrol goes to a step 905 where the "R" range target turbine revolutionspeed TRPM is determined by retrieval of a "R" range target turbinerevolution speed table. Subsequently to the determination of the targetturbine revolution speed TRPM by retrieval of the target turbinerevolution speed table at the step 902 or 904 or 905, the controlproceeds to a step 906 where the vehicle speed V_(S) is read in the RAM.At a subsequent step 908, a current rotary position θs (theta s) of theshift motor 152 is calculated based on the target turbine revolutionspeed TRPM and the vehicle speed V_(S). This current rotary position θs(theta s) indicates an amount of feed-forward control. Then, a currentturbine revolution speed N_(T) is read in the RAM at a step 910, and thedeviation e between the target turbine revolution speed TRPM and thecurrent revolution speed N_(T) is calculated at a step 912. At asubsequent step 914, it is determined whether an absolute value of thedeviation e is less than or equal to a second predetermined value C₂ ornot. This second predetermined value C2 is set equal to 300 rpm, forexample. If |e|<C₂, the control proceeds to a step 916 where thedeviation e is set to e₁, and it goes from the step 916 to a step 952 asshown in FIG. 8. On the other hand, at the step 914, if |e|≧C₂, thecontrol proceeds to a step 918 where it is determined whether thedeviation e is more than or equal to 0 (zero) or not. If e>0, thecontrol goes to a step 920 where C₂ is set to e₁. On the other hand, ife<0, the control goes to a step 922 where --C₂ is set to e₁.Subsequently to the step 920 or 922, the control proceeds to the step952 as shown in FIG. 8.

Referring to FIG. 8, at the step 952, a current engine torque T_(E) isread in the RAM. At a step 954, two constants K₁, K₂ are set inproportion to T_(E). It is to be noted that values of K₁, K₂ are largerthan 1, respectively, and set to be increased in proportion to T_(E) asshown in FIGS. 10, 11. At a step 956, two products are obtained bymultiplying two predetermined values K_(po), K_(io) by the constants K₁,K₂, respectively, and set as K_(p), K_(i). At a subsequent step 924, aproduct of e₁ and K_(p) is set as P. This product P indicates aproportional term of an amount of feed-back control which is variable inproportion to the deviation e. Subsequently, at a step 926, it isdetermined whether the absolute value of the deviation e is less than orequal to a first predetermined value C₁ or not. This first predeterminedvalue C₁ is set equal to 500 rpm, for example. If |e|≦C₁, the controlproceeds to a step 928 where an integral term is obtained by multiplyingan integral of e₁ by the constant K_(i), and set as I. On the otherhand, if |e|>C₁, the control proceeds to a step 930 where I is set to 0(zero). That is, an integrator is reset to zero. Subsequently to thestep 928 or 930, the control goes to a step 932 where a sum of I and Pis set as D_(pi). This sum D_(pi) indicates the amount of feed-backcontrol. A sum of θs (theta s) (the rotary position) and D_(pi) is set,at a step 934, as the target pulse number P_(D). At a subsequent step936, it is determined whether the target pulse number P_(D) is less than0 (zero) or not. Zero pulse number corresponds to the maximum reductionratio which can be established in the continuously variabletransmission. If P_(D) >0, the control proceeds to a step 938 whereaddition of the integral value is discontinued. Subsequently, at a step940, the target pulse number P_(D) is set to 0 (zero), then the controlgoes to the step 630 as shown in FIG. 9. On the other hand, at the step936, if P_(D) ≧0, the control proceeds to a step 942 where it isdetermined whether P_(D) is more than or equal to a predetermined valueH_(i). This predetermined value H_(i) is a pulse number corresponding toa minimum reduction ratio which can be established in the continuouslyvariable transmission. If the P_(D) <H_(i), the control proceeds fromthe step 942 to the step 930 as shown in FIG. 9. If P_(D) >H_(i), thecontrol proceeds to a step 944 where addition of the integral value isdiscontinued. Subsequently, the control goes to a step 946 where thetarget pulse number P_(D) is set to the predetermined value H_(i), thenit goes to the step 630.

At the step 630, a comparison is made between the target pulse numberP_(D) and the current pulse number P_(A). If P_(D) =P_(A), the shiftmotor drive signal is outputted at a step 636, and the solenoid drivesignal is outputted at a step 638, then the control is returned toSTART. If P_(A) <P_(D), the shift motor drive signal is displaced, at astep 632, in an upshift direction. At a subsequent step 634, a sum ofthe current pulse number P and 1 is set as the new pulse number P_(A),then the control proceeds to the step 636. If P_(A) >P_(D), the shiftmotor drive signal is displaced, at a step 620, in a down-shiftdirection. At a subsequent step 622, a difference between the currentpulse number P_(A) and 1 is set as the new pulse number P_(A), then thecontrol proceeds to the step 636.

In brief, the following control is carried out according to theabove-mentioned routine. First, at the step 908, the amount offeed-forward control is calculated. At the step 924, the proportionalterm P of the amount of feed-back control is calculated, then, at thestep 928, the integral term I of the feed-back control is calculated.Subsequently, at the step 632, the amount of feed-back control D_(pi) isgiven by the sum of the integral term I and the porportional term P. Inthis event, feed-back gains are k₁ times and K₂ times the referencevalues (K_(po), K_(io)), respectively. As described hereinbefore, thevalues of K₁, K₂ become greater as the engine torque T_(E) increases.Thus, the larger is the engine torque T_(E), the higher are theresponsibility and the followability of the shift control. Accordingly,even if the engine torque T_(E) is large, quick responsibility ofshifting can be achieved. Further, during normal operation of the motorvehicle, the current reduction ratio is inhibited from exceeding thetarget reduction ratio.

Having described the embodiment wherein the present invention is appliedto the continuously variable traction roller transmission, it is to benoted that the present invention is also applicable to a continuouslyvariable V-belt transmission.

What is claimed is:
 1. An apparatus comprising:a continuously variabletransmission having at least one pair of traction rollers; and a systemfor controlling said continuously variable transmission such that thecontinuously variable transmission is adjusted to a reduction ratio thatis determined in response to a position at which a shift actuator ispositioned, the system comprising means for determining a target valueof a predetermined variable related to a reduction ratio which thecontinuously variable transmission should be adjusted to and forgenerating a target value indicative signal indicative of said targetvalue determined; means for detecting a current value of saidpredetermined variable and for generating a current value indicativesignal indicative of said current value detected; means for detecting acurrent torque which the continuously variable transmission is subjectto and for generating a current torque indicative signal indicative ofsaid current torque detected; means for controlling the shift actuatorin a feed-back control manner with a feed-back gain such that adeviation between said current value indicative signal and said targetvalue indicative signal reduces toward zero; means for altering saidfeed-back gain in response to said current torque indicative signal; andshift control valve means for controlling a distribution of hydraulicpressure related to said reduction ratio in response to the position atwhich the shift actuator is positioned.
 2. A method of controlling acontinuously variable transmission such that the continuously variabletransmission is adjusted to a reduction ratio that is determined inresponse to a position at which a shift actuator is positioned, thecontinuously variable transmission having at least one pair of tractionrollers, the method comprising the steps of:determining a target valueof a predetermined variable related to a reduction ratio which thecontinuously variable transmission should be adjusted to and generatinga target value indicative signal indicative of said target valuedetermined; detecting a current value of said predetermined variable andgenerating a current value indicative signal indicative of said currentvalue detected; detecting a current torque which the continuouslyvariable transmission is subject to and generating a current torqueindicative signal indicative of said current torque detected;controlling the shift actuator in a feed-back control manner with afeed-back gain such that a deviation between said current valueindicative signal and said target value indicative signal reduces towardzero; altering said feed-back gain in response to said current torqueindicative signal; and controlling a distribution of hydraulic pressurerelated to said reduction ratio in response to the position at which theshift actuator is positioned.
 3. The apparatus of claim 1, wherein saidmeans for altering said feed-back gain includesmeans for setting firstand second constants in proportion to said current torque detected, andmeans, responsive to said means for setting, for altering proportionaland integral components of said feed-back gain using said first andsecond constants, respectively.
 4. The apparatus of claim 2, whereinsaid step of altering said feed-back gain includessetting first andsecond constants in proportion to said current torque detected, andaltering proportional and integral components of said feed-back gainusing said first and second constants, respectively.